What Causes Chronic Diarrhea And Weight Loss?

My husband, who is 62 years old, has had diarrhea for 5-6 months at least once a day and has lost about 25 lbs. He is being treated by a gastroenterologist and had a colonoscopy, which he said was perfectly normal. The first thing the doctor had him do was go off of Metformin and dairy products for two weeks and then get back to him. There was no change, so he was put on Lomotil 1 pill twice a day to slow down the diarrhea and it had no affect. Then he was prescribed Viberzi, which had no affect. The doctor then wanted to put him on Xifaxan, which took a few weeks to get approved through our insurance without having to pay a $700 co-pay. He started taking the Xifaxan and now he s worse than ever. The diarrhea is now completely watery and 3-4 times a day. He s had to stop taking it and take the Lomotil 1-2 pills every 4 hours until it stops. My husband is about at the end of his rope and dressed with this. He s afraid to leave the house most of the time. Do you have any idea what could be causing this and any suggestions about what to do? His gastroenterologist is retiring at the end of the year...maybe that s a good thing?